When Did the Earliest Humans Reach North America?

Paleontologists examining mastodon bones found in the San Diego area in 1992 have recently proposed that what we know about early humans in North America may be all wrong. In a study published this year in the journal Nature, the research team claims that they see evidence of early man in North America dating back to around 130,000 years ago. Previous research had pegged the arrival of early man on the continent to around 24,000 years ago, so the much-earlier date is controversial, to say the least. The new research is based on the fact that the mastodon bones -- including two tusks, three molars, 16 ribs, and more than 300 bone fragments -- show impact marks, suggesting that they were struck with hard objects. In fact, the researchers also found five “hulking stones,” which they say could have been used as hammers and anvils.

The mystery of the mastodon bones:

• Two distinct clusters of broken mastodon bones surrounded the stones, suggesting to the researchers that the bones were smashed in that location.
• Some of the shattered bones contained spiral fractures, indicating that they were broken while still “fresh,” the authors wrote.
• Attempts at radiocarbon dating were unsuccessful because the bones did not have enough carbon-containing collagen. Instead, the researchers used uranium-thorium dating -- a technique often used to check the dates derived from radiocarbon dating.